This invention relates generally to the treatment of contaminated liquids with oxidizing gases, and more particularly to a method of improving the quality of diesel fuel by removing sulfur compounds therefrom and raising the cetane number thereof.
Domestic diesel fuel contains, at most, 0.05% sulfur by weight. Refineries produce diesel fuel as a distillate of crude oil which is then hydrotreated to reduce the sulfur content to 0.05% or less. In hydrotreating, a reducing process, the diesel fuel is contacted with high pressure hydrogen gas in the presence of a solid catalyst. The catalyst serves to remove the sulfur in the form of sulfide, while the hydrogen serves to saturate the molecules from which the sulfur is removed.
While hydrotreating is effective at reducing the sulfur content of diesel fuels substantially, certain forms or sulfur are easier to remove then others. For example, sulfur in the form of sulfide is easier to remove than sulfur in the thiophene form. As a result, the sulfur that remains in diesel fuel after hydrotreating is mostly in the thiophene form. As thiophenes increase in complexity, the sulfur becomes more difficult to remove, requiring hydrotreating at more severe conditions. The higher the severity of the hydrotreating process, the shorter catalyst life time. Additionally, severe hydrotreating conditions lead to cracking of the diesel fuel molecules and thus higher yield losses. The sulfur remaining in diesel fuel after hydrotreating is primarily or exclusively in the thiophene form. Thiophenes reduced during the hydrotreating process are converted to biphenyl derivatives.
Recently, the United States Environmental Protection Agency (EPA) promulgated regulations sharply reducing the allowed sulfur content of diesel fuels. No later than year 2006, diesel fuel must contain less than 15 ppm sulfur (0.0015% by weight). This standard will require hydrotreating under conditions of increased severity.
Diesel fuel quality is rated by cetane number, a parameter similar to the octane rating number for gasoline. The higher the cetane number, the higher the quality of the diesel fuel. One class of compounds that contributes to a low cetane number are aromatics, including biphenyl derivatives. Thus, when hydrotreating is used to remove sulfur from diesel fuel, thiophenes are converted to biphenyl derivatives and the cetane number of the fuel is reduced.
In accordance with the present invention, sulfur in diesel fuel is oxidized to a sulfoxide or a sulfone. Once in the sulfoxide or sulfone form, the sulfur-containing molecule is easily removed from the diesel fuel by distillation or extraction. In contrast to hydrotreating, oxidative desulfurization and the subsequent removal of the molecules containing the oxidized sulfur results in a decrease in aromatic content. The decrease in aromatic content leads directly to an increase in cetane number.
In accordance with the one aspect of the invention, oxidizing gas is utilized at its source and is formed into sub-micron size bubbles which are immediately dispersed into flowing diesel fuel. Due to the sub-micron size of the bubbles, the surface area of the oxidizing gas is greatly increased, thereby greatly increasing the efficiency of the oxidation reaction.
In accordance with another aspect of the invention, the oxidizing gas is formed into sub-micron size bubbles by directing it through a sintered glass, sintered ceramic, or porous ceramic tube. Diesel fuel is caused to flow past the exterior of the sintered/porous tube. The flowing diesel fuel cleaves sub-micron size bubbles of the oxidizing gas from the surface of the tube. The sub-micron size bubbles of oxidizing gas are dispersed into the diesel fuel, whereupon sulfur compounds contained within the diesel fuel are efficiently oxidized into sulfoxides or sulfones, which are in turn removed by distillation or extraction.